refactor ECDH multiplication into ecdh_multiply function

bip32.c

@@ -470,20 +470,9 @@ int hdnode_get_shared_key(const HDNode *node, const uint8_t *peer_public_key, ui
 		*result_size = 33;
 		return 0;
 	} else {
-		curve_point point;
-		const ecdsa_curve *curve = node->curve->params;
-		if (!ecdsa_read_pubkey(curve, peer_public_key, &point)) {
+		if (!ecdh_multiply(node->curve->params, node->private_key, peer_public_key, session_key)) {
 			return 1;
 		}
-		bignum256 k;
-		bn_read_be(node->private_key, &k);
-		point_multiply(curve, &k, &point, &point);
-		MEMSET_BZERO(&k, sizeof(k));
-
-		session_key[0] = 0x04;
-		bn_write_be(&point.x, session_key + 1);
-		bn_write_be(&point.y, session_key + 33);
-		MEMSET_BZERO(&point, sizeof(point));
 		*result_size = 65;
 		return 0;
 	}

ecdsa.c

@@ -629,6 +629,26 @@ void scalar_multiply(const ecdsa_curve *curve, const bignum256 *k, curve_point *
 
 #endif
 
+int ecdh_multiply(const ecdsa_curve *curve, const uint8_t *priv_key, const uint8_t *pub_key, uint8_t *session_key)
+{
+	curve_point point;
+	if (!ecdsa_read_pubkey(curve, pub_key, &point)) {
+		return 1;
+	}
+
+	bignum256 k;
+	bn_read_be(priv_key, &k);
+	point_multiply(curve, &k, &point, &point);
+	MEMSET_BZERO(&k, sizeof(k));
+
+	session_key[0] = 0x04;
+	bn_write_be(&point.x, session_key + 1);
+	bn_write_be(&point.y, session_key + 33);
+	MEMSET_BZERO(&point, sizeof(point));
+
+	return 0;
+}
+
 // generate random K for signing
 void generate_k_random(bignum256 *k) {
 	int i;

ecdsa.h

@@ -67,6 +67,7 @@ int point_is_infinity(const curve_point *p);
 int point_is_equal(const curve_point *p, const curve_point *q);
 int point_is_negative_of(const curve_point *p, const curve_point *q);
 void scalar_multiply(const ecdsa_curve *curve, const bignum256 *k, curve_point *res);
+int ecdh_multiply(const ecdsa_curve *curve, const uint8_t *priv_key, const uint8_t *pub_key, uint8_t *session_key);
 void uncompress_coords(const ecdsa_curve *curve, uint8_t odd, const bignum256 *x, bignum256 *y);
 int ecdsa_uncompress_pubkey(const ecdsa_curve *curve, const uint8_t *pub_key, uint8_t *uncompressed);